Traction control systems, also known as anti-slip regulation (ASR) systems, are used on vehicles to prevent loss of traction of the driven wheels so as to maintain control of the vehicle and to prevent deterioration of the accelerating performance due to longitudinal slip of the driving wheels. Traction control is necessary, for example, when excessive accelerator input is applied by the driver and the condition of the road surface is unable to cope with the torque applied.
Feedback control methods are typically employed in such systems, whereby the slip of the driving wheels (wheel slip) is monitored and, when it becomes excessive due to an excessive driving torque (e.g. sudden application of the accelerator pedal), appropriate action is taken to reduce the engine output and/or to apply a braking force to the driving wheels. Traction control may be implemented within the engine control unit (ECU) of the vehicle to reduce engine torque by retarding or suppressing the spark to one or more cylinders of the engine, reducing fuel supply to one or more of the engine cylinders, closing the throttle, or, in turbo-charged vehicles, actuating the boost control solenoid to reduce boost and therefore engine power. Additionally, the wheel brake to one or more of the wheels may be applied to control wheel slip.
Traction control systems are typically implemented in a vehicle as part of a stability control system (SCS) operable to enhance stability of a vehicle by detecting and reducing skidding. If a skid is detected whilst cornering, the stability control system is arranged automatically to apply braking to individual wheels to assist a driver in steering the vehicle in an intended direction, for example around a corner.
A problem has been found with existing traction control systems that accurate estimation or measurement of vehicle speed using currently known techniques can be unreliable, particularly in four wheel drive vehicles with high drive torque capability. Electric propulsion motors offer particularly high values of drive torque for motor vehicle applications, and are capable of delivering relatively high rates of change of torque. It is found that current stability control technologies may be inadequate to maintain vehicle stability in some situations, for example when the driver demands high acceleration from rest.
For known stability control systems it is of fundamental importance to know the speed of the vehicle at any given moment in time. Speed may be measured by one of a number of different techniques, including measurement of wheel speed by means of one or more wheel speed sensors. In the case of a two wheel drive vehicle, measurement of speed of rotation of the non-driven wheels typically enables a reliable estimation of vehicle speed to be made at any given moment in time based on measurement of the speed of rotation of the non-driven wheels even if the driven wheels are experiencing excessive slip.
However, in the case of a four wheel drive vehicle, under conditions of harsh acceleration it is possible that each of the four wheels experiences excessive slip such that a wheel speed measurement will indicate that the vehicle speed is greater than it actually is. This can result in inappropriate behaviour of a stability control system.
It is an object of the invention to provide an improved control system for a vehicle which addresses one or more of the aforementioned problems.